In general, a backlight unit for LCD and for advertisement or illumination comprises a light guide panel made of a transparent acrylate board. The light guide panel has prominences and depressions in the form of dots or grooves on one side thereof to induce refraction and scattering of light, and a light source provided at one side end of the light guide panel.
FIG. 1 is a rear view of a backlight unit (100) according to the conventional technology, and FIG. 2 is a partial sectional view of FIG. 1 along the line A—A.
Referring to FIGS. 1 and 2, a light source (120) for radiating light is provided at one side end of a light guide panel (110). The light guide panel (110) is made of translucent material, and its front side has a display film (140) attached thereto, while its backside is provided with a plurality of horizontal and vertical V-shaped grooves (112) to uniformly scatter and refract the light emitted from the light source to the front side and is covered with a reflection sheet (150), such as polyethyleneterephthalate (PET) material.
On the backside of the conventional light guide panel (110), the intervals between two adjacent lines of the V-shaped grooves (112) gradually become narrow in order to obtain uniform brightness through the light guide panel (110). This is because the closer to the light source (120), the less the light is lost during the course of transmission. That is, since the light from the light source (120) is not lost as readily when it is closer to the light source (120), the V-shaped grooves (112) near the light source (120) can be arranged at broader intervals than the V-shaped grooves (112) distant from the light source (120). If the size of the light guide panel (110) increases, the size of the light source (120) must also be increased to maintain a certain brightness.
Another conventional light guide panel is manufactured by coating an acrylate board with light diffusing material without V-shaped grooves or prominences and depressions in the form of dots. This is focused on mass-production in lower cost, rather than on provision of uniform brightness. Accordingly, the closer to the light source, the brighter it is, while the more distant from the light source, the darker it is. In this case also, if the size of the light guide panel increases, the size of the light source must also be increased to maintain a certain brightness.
In the above conventional light guide panels, the thickness of the light guide panel should correspond to the diameter of the light source in order to absorb, refract, and scatter the light emitted from the light source as much as possible. However, although a certain level of brightness can be obtained, the conventional light guide panels have the following disadvantages:
As the width and thickness of the light guide panel must be increased in proportion to the size of the light source, the increased volume and weight of the light guide panel complicate handling and installation, increase manufacturing costs, while the diminishing performance.
As an alternative for these disadvantages, as shown in FIG. 3, reflection sheets (130) were provided to cover the light source (120), and thus the light guide panel (110) having a thickness less than the diameter of the light source (120) has been used. In this case, the light guide panel (110) could be lighter than the above conventional ones and could be easily installed with a lower cost. However, the brightness remarkably deteriorated much more than the case in FIG. 2 where the light guide panel is as thick as the diameter of the light source. Thus, such a light guide panel (110) was mainly used for small-sized advertising boards only.
U.S. Pat. No. 6,068,382 entitled “Panel-Form Illuminating System” illustrates a light guide panel wherein a light radiating part is inclined in the form of stairs to become thinner and thinner as it becomes distant from a light source.
The principle of this US patent is in the stair structure to induce refraction and scattering of light by forming V-shaped grooves and protrusions in the inclined parts of the panel. The thickness of the panel at its both side ends and the degree of inclination focuses on the uniform brightness.
In order to obtain the uniform brightness, however, the panel has to be manufactured by an injection molding, by which the size and use of the panel is to be limited. Therefore, the panel can be used only for light guide panels of notebook computers, and cannot be used for middle or large-sized light guide panels.
Also, the roller pressing renders the production of middle and large-sized panels practically impossible due to the following reasons:
Firstly, the panel must be molded while passing through between two inclined rollers, but it is difficult to supply raw materials of the panel in proportion to its varying thickness.
Secondly, as the light source differs in pattern and slope according to the lightness, the light source is limited.
Thirdly, many rollers are required since the slopes differ depending on the width of the panel.
Fourthly, in order to install the panel on the wall or the plane, it is necessary to fill the difference between the neighboring stairs with supplement.
Currently, light guide panels for LCD monitors and advertising boards are made of mass-productive acrylate boards, which are superior to the above prior technology, for example, by enabling to be cut in various sizes.